These so-called "URL shortening" services are absolutely trivial to implement. Twitter (which, since it sends out SMS messages containing links, is one of the few environments where "shortened" URLs are actually worth bothering with at all) could have rolled their own and protected their users from one source of failure that they are powerless to prevent. When the social networking bubble bursts people are going to regret the time they invested building up their "presence" on web sites that don't exist any more.